Plant cell wall is the only source of cellulose for the paper industry and is a promising source of sugar for lignocellulosic biofuels. The utilization of plants to convert solar energy into transportable and storable energy will have a positive impact on the environment, since using plants can help to drastically reduce the utilization of fossil-derived fuels, can reduce carbon emission into the atmosphere, and even can contribute to carbon sequestration. However, even if lignocellulosic biofuels will be beneficial for the environment, the cost to produce them is still not cost-effective, mainly due to the expensive raw sugar derived from plant cell wall. The low density, recalcitrance to enzymatic hydrolysis, and low ratio of hexoses to pentoses in the biomass are the main contributors to the sugar cost because they impact transportation cost and require high amount of energy and chemicals. Therefore, improving the digestibility of the raw biomass and improving recovery of sugars that are more readily fermentable from biomass will have an important beneficial impact on the cost of lignocellulosic biofuels production.
Plant cell walls are predominantly composed of different polysaccharides, which can be grouped into cellulose, hemicelluloses and pectin. Pectin is a class of polysaccharide characterized by a high content of galacturonic acid residues and consists of two major types: homogalacturonan entirely composed of alpha-1,4-linked galacturonosyl residues, and rhamnogalacturonan I (RGI) composed of a backbone of alternating rhamnose and galacturonic acid residues with sidechains composed of arabinan and beta-1,4-galactan (Mohnen, 2008; Harholt et al., 2010). Other domains of pectin include RGII, a complex structure with numerous different sugars, and xylogalacturonan, which is a type of HG with sidechains consisting of single xylosyl residues. It has been estimated that as many as 67 different transferases are required for bioynthesis of pectin (Mohnen, 2008; Harholt et al., 2010) but so far only one has been unambiguously indentified, namely the homogalacturonan galacturonosyltransferase GAUT1 (Sterling et al., 2006). A likely xylogalacturonan xylosyltransferase designated XGD1 has also been described but the final proof of activity of the isolated XGD1 protein has not been provided (Jensen et al., 2008). Biosynthesis of the arabinan sidechains on RGI involves the ARAD1 and ARAD2 proteins, but if they are arabinosyltransferases or work in a different way has not been determined (Harholt et al., 2006; Harholt et al., 2012). Beta-1,4-galactan constitutes a large part of pectin and of the total cell wall. However, little is known about the enzymes in plants responsible for its synthesis.